The present invention relates to the formation of substantially low density inorganic foams having good compressive strength and cell structure which renders them suitable for insulating and structural utility.
Inorganic foams, especially those formed from alkali metal silicates, are known fire-resistant materials which have excellent insulation properties. The thermal conductivity of these foams compares favorably with foams made of polystyrene or of polyurethane. Further, such foams are resistant to decomposition by exposure to high temperatures, moisture or a combination thereof. The desirability of utilizing inorganic foams, especially those formed from alkali metal silicates, as a structural insulation is well documented.
Although extensive work has been previously done in attempts to form desirable silicate foams, these attempts have generally resulted in processes requiring certain parameters which cause the resultant foam to be economically uncompetitive with other insulating materials presently in the marketplace.
For example, U.S. Pat. No. 3,396,112 to Burrows discloses the formation of silicate foam by mixing dry, particulate alkali metal silicate with aluminum powder and subsequently contacting the mixtures with water. This process and those which similarly use dry starting materials, are unsuitable for commercialization because the starting materials are expensive and the amount of dry aluminum powder required renders the finished product economically prohibitive.
U.S. Pat. No. 3,700,470 to Barton attempts to overcome the economic barrier of using large amounts of dry aluminum powder by producing light weight foamed materials from dry starting components which are admixed with a substantial amount of a ceramic filler. Although this process is economically more competitive, it still requires the utilization of a dry alkali metal silicate which causes increased cost of the raw materials used.
The utilization of the less expensive commercially available aqueous solutions of alkali metal silicate, such as water glass solutions, have been attempted without substantial success. Such solutions contain dissolved silicon oxide which forms a protective coating on the surface of aluminum and thereby renders the aluminum inert to attack and unable to generate hydrogen gas. Presently known processes which use water glass solutions require the additional utilization of a blowing agent to produce a foamed object. Further, foams produced in this manner have a high degree of unreacted aluminum in the resultant product which may cause them to have inferior strength. Although the strength of these foams have been increased by the incorporation of various additives into the starting material the use of such additives and of a blowing agent add to the cost of producing a product which must compete in a cost conscious market.